An electric power actuator, which is operated by electric power, can be intended particularly, yet not only, to actuate an aircraft control surface, specially in a transport aircraft, for example an elevator, a rudder or an aileron of an aircraft.
This electric power actuator can be of the electro-hydrostatic actuator (EHA) type which usually comprises an electronic unit, an electric motor, a hydraulic pump, a pressure relief valve, a hydraulic block and a hydraulic jack. This actuator is controlled by a control current sent to the electronic unit. A local closed-loop control in the electronic unit converts this control current into a velocity set point for the electric motor. The latter drives the hydraulic pump using an electric power provided by the aircraft. The pump then locally generates hydraulic power for moving the hydraulic jack.
This invention can also be applied to an electric backup hydraulic actuator (EBHA) type, which is a hybrid actuator comprising both characteristics of a usual hydraulic servo-control and electro-hydrostatic actuator of the EHA type. In a rated situation (out of breakdown), the EBHA actuator operates as a usual servo-control. By contrast, in case of a breakdown affecting the hydraulic mode, this EBHA actuator switches to electric power supply and operates as an EHA actuator.
With such a technology, an EHA actuator is capable of generating locally more strain than the maximum level necessary for being operated. There is therefore a risk that the structure on which the EHA actuator is mounted be subjected to higher strains than the level for which it is sized. Thus, in order to protect this structure, EHA actuators are generally equipped with at least one pressure relief valve which limits strain generated by the actuator.
It should be noticed that in some configurations, the EHA actuator can be caused to operate on the pressure relief valve. In this case, the motor rotates and the hydraulic pump generates a flux which flows in the pressure relief valve instead of feeding the chambers of the jack.
In such a situation that is with a sustained flow of fluid between the pump and the pressure relief valve the actuator EHA can be very quickly damaged to the point where it can not be used anymore.
There are two different possible causes for the actuator breakdown:                a fluid overheating. The fluid which flows between the hydraulic pump and the pressure relief valve flows through a little port when it flows through the pressure relief valve, with the result that it is very quickly heated. This heat built up in the fluid is then dissipated in the whole actuator. The resulting temperature rise can quickly lead to a damaged actuator; and        an overheating of the control electronics. When the pressure relief valve is opened and the fluid flows therethrough, the electric motor must provide a high torque for, on the one hand, maintaining the pressure difference in the chambers of the jack, and on the other hand, maintaining a constant speed of the actuator. In order to maintain this high torque, a current, which is also high, must flow in the electronic unit of the actuator. As the electronics of the actuator are not sized for withstanding such current level, they can quickly be damaged.        